Are you actually running into performance issues?
I think Box2D already does many optimizations, not sure about this one in particular, though. The documentation on
SetSpritePhysicsOn states:
Quote: "Once you have called this command to determine the mode of the sprite, further calls will ignore the mode values and it will only reactivate a sprite that has been turned off, as the mode cannot be changed whilst the sprite is in the physics system."
Maybe you could turn physics off/on manually when they are off-screen?